What happens?
The patella (kneecap) normally tracks in a shallow groove on the front of the femur called the trochlea. When the patella dislocates, it usually slips outwards (laterally) out of the groove — often during a twisting movement or a direct blow.
Stability factors
Several anatomical features influence patella stability:
- The shape and depth of the trochlear groove
- The medial patellofemoral ligament (MPFL), which acts as a "checkrein" preventing lateral movement
- Quadriceps balance — particularly the vastus medialis obliquus (VMO)
- Generalised ligament laxity
- Valgus alignment ("knock knees")
Diagnosis
A clinical examination is usually diagnostic. Investigations include:
- Standard X-rays of the knee
- Long-leg standing X-ray to assess overall alignment
- MRI to look at the MPFL, cartilage and any bone bruising
- CT in some cases to measure the relationship between the tibial tuberosity and the trochlear groove (TT-TG distance)
Non-surgical treatment
After a first dislocation, around 60–70% of patients heal without further dislocations following appropriate non-surgical management:
- Initial period in a splint or brace
- Physiotherapy with quadriceps and VMO strengthening
- Patellar taping for return to sport
Surgical treatment
- Arthroscopic removal of cartilage or bone fragments after dislocation
- Open repair when a large osteochondral fragment is displaced
- Ligament reconstruction — see patella stabilisation — for recurrent instability